A Simple, Accurate Fluoroscopic Method of Foreign Body Localization

Abstract

The method of foreign body localization herein described has proved in practice to be highly accurate and rapidly and easily accomplished. It has the added advantage of requiring no special apparatus. It is applicable to any fluoroscopic unit having some provision for fixing the distance from fluoroscopic screen to table top. The patient is placed on the fluoroscopic table. The fluoroscopic screen is fixed at a predetermined distance from the table top, and its illumination is limited to a narrow beam centered directly over the foreign body to be localized. A lead cross or other metallic marker is affixed by adhesive tape to the patient's skin on the dependent side in such a position that the projected images of the foreign body and cross are superimposed. The fluoroscopic beam is then adjusted to a narrow slit parallel to the long axis of the body, extending' across the full diameter of the screen. The screen is moved cranially until the center of the image of the skin marker coincides with the edge of the screen. Then, with a soft wax pencil, a mark is made on the screen at the center of the projected foreign body image. The screen is next shifted caudally until the shadow of the skin marker coincides with the opposite margin of the screen. A second mark is made with the wax pencil at the new position of the foreign body image. The distance between wax marks on the fluoroscopic screen is then measured, and, by reference to a calibration chart, the depth of the foreign body can be read off directly. The point of attachment of the lead marker is indicated on the skin by wax pencil at the time the marker is removed. The position of the foreign body can then be reported as being “— cm, deep to the mark on the skin.” Localization depends on the relative shift of the projected foreign body image on the fluoroscopic screen as compared with that of the lead skin marker, which, being attached to the dependent portion of the patient's body, is at table-top level. By using the diameter of the fluoroscopic screen as a measure, the skin marker image shift is kept constant. The screen-table top and focal-screen distances are also fixed. Thus, the only variable factor is the shift of the foreign body image, which, in turn, is proportional to depth of the foreign body in the tissues. A calibration chart indicating depth of foreign body in relation to foreign body image shift can be constructed rnathematically, given focal-table top, focal-foreign body, and focal-screen distances. More simply, calibration can be made by using a phantom, such as a discarded film box top, to which appropriate lead numbers are fixed at 1 cm. intervals from fluoroscopic table.